The United States has pledged to tackle the Syrian-backed Hizbollah group in the next phase of its 'war on terror' in a move which could threaten military action against President Bashar Assad's regime in Damascus.

The move is part of Washington's efforts to persuade Israel to support a new peace settlement with the Palestinians. Washington has promised Israel that it will take 'all effective action' to cut off Syria's support for Hizbollah - implying a military strike if necessary, sources in the Bush administration have told The Observer .

Hizbollah is a Shia Muslim organisation based in Lebanon, whose fighters have attacked northern Israeli settlements and harassed occupying Israeli troops to the point of forcing an Israeli withdrawal from southern Lebanon three years ago.

The new US undertaking to Israel to deal with Hizbollah via its Syrian sponsors has been made over recent days during meetings between administration officials and Israeli diplomats in Washington, and Americans talking to Israeli Prime Minister Ariel Sharon in Jerusalem. It would be part of a deal designed to entice Israel into the so-called road map to peace package that would involve the Jewish state pulling out of the Palestinian West Bank, occupied since 1967.

Prime Minister Ariel Sharon has so far rejected the road map initiative - charted by the US with its ally, Britain - which also calls for mutual recognition between Israel and a new Palestinian state, structured according to US-backed reforms. The American guarantee would be to take armed action if necessary to cut off Syrian support for Hizbollah, and stop further sponsorship for the group by Iran.

'If you control Iraq, you can affect the Syrian and Iranian sponsorship of Hizbollah, both geographically and politically,' says Ivo Daalder of the Brookings Institution think-tank in Washington.

'The United States will make it very clear, quietly and publicly, that Baathist Syria may come to an end if it does not stop its support of Hizbollah.'

The undertaking dovetails conveniently into 'phase three' of what President George Bush calls the 'war on terror' and his pledge to go after all countries accused of harbouring terrorists.

It also fits into calls by hawks inside and aligned to the administration who believe that war in Iraq was first stage in a wider war for American control of the region. Threats against Syria come daily out of Washington.

Hawks in and close to the Bush White House have prepared the ground for an attack on Syria, raising the spectre of Hizbollah, of alleged Syrian plans to wel come refugees from Saddam Hussein's fallen regime, and of what the administration insists is Syrian support for Iraq during the war.

Deputy Defence Secretary Paul Wolfowitz - regarded as the real architect of the Iraqi war and its aftermath - said on Thursday that 'the Syrians have been shipping killers into Iraq to try and kill Americans', adding: 'We need to think about what our policy is towards a country that harbours terrorists or harbours war criminals.

'There will have to be change in Syria, plainly,' said Wolfowitz.

Washingtom intelligence sources claim that weapons of mass destruction that Saddam was alleged to have possessed were shipped to Syria after inspectors were sent by the United Nations to find them.

One of the chief ideologists behind the war, Richard Perle, yesterday warned that the US would be compelled to act against Syria if it emerged that weapons of mass destruction had been moved there by Saddam's fallen Iraqi regime.